1. Field of the Invention
The present invention relates generally to the field of emission control equipment for boilers, heaters, kilns, or other flue gas-, or combustion gas-, generating devices (e.g., those located at power plants, processing plants, etc.) and, in particular to a new and useful method and apparatus having an increased efficiency in the ability to control the emission of NOx without a simultaneous increase in the amount of ammonia slip.
2. Description of the Related Art
NOx refers to the cumulative emissions of nitric oxide (NO), nitrogen dioxide (NO2) and trace quantities of other nitrogen oxide species generated during combustion. Combustion of any fossil fuel generates some level of NOx due to high temperatures and the availability of oxygen and nitrogen from both the air and fuel. NOx emissions may be controlled using low NOx combustion technology and post-combustion techniques. One such post-combustion technique is selective catalytic reduction using an apparatus generally referred to as a selective catalytic reactor or simply as an SCR.
SCR technology is used worldwide to control NOx emissions from combustion sources. This technology has been used widely in Japan for NOx control from utility boilers since the late 1970's, in Germany since the late 1980's, and in the US since the 1990's. The function of the SCR system is to react NOx with ammonia (NH3) and oxygen to form molecular nitrogen and water. Industrial scale SCRs have been designed to operate principally in the temperature range of 500° F. to 900° F., but most often in the range of 550° F. to 750° F. SCRs are typically designed to meet a specified NOx reduction efficiency at a maximum allowable ammonia slip. Ammonia slip is the concentration, expressed in parts per million by volume, of unreacted ammonia exiting the SCR.
Selective non-catalytic reduction, SNCR, is a related technology where ammonia and NOx react in a homogeneous gas phase environment to produce molecular nitrogen and water vapor. This system must operate at higher temperatures than the SCR systems. Typical operating temperatures range from 1800° F. down to 1500° F. This technology is generally applied to fluidized bed combustion applications that produce highly alkaline fly ashes. The ammonia slip in these applications is generally higher than it is in SCR applications.
For additional details concerning NOx removal technologies used in the industrial and power generation industries, the reader is referred to Steam/its generation and use, 41st Edition, Kitto and Stultz, Eds., Copyright© 2005, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., particularly Chapter 34—Nitrogen Oxides Control, the text of which is hereby incorporated by reference as though fully set forth herein.
Recent regulations (March 2005) issued by the EPA promise to increase the portion of utility boilers equipped with SCRs. SCRs are generally designed for a maximum efficiency of about 90%. This limit is not set by any theoretical limits on the capability of SCRs to achieve higher levels of NOx destruction. Rather, it is a practical limit set to prevent excessive levels of ammonia slip. This problem is explained as follows.
In an SCR, ammonia reacts with NOx according to one or more of the following stoichiometric reactions (a) to (c):4NO+4NH3+O2→4N2+6H2O  (a)4NO2+4NH3→4N2+6H2O+O2  (b)2NO2+4NH3+O2→3N2+6H2O  (c).
The above reactions are catalyzed using a suitable catalyst. Suitable catalysts are discussed in, for example, U.S. Pat. Nos. 5,540,897; 5,567,394; and 5,585,081 to Chu et al., all of which are hereby incorporated by reference as though fully set forth herein. Catalyst formulations generally fall into one of three categories: base metal, zeolite and precious metal.
Base metal catalysts use titanium oxide with small amounts of vanadium, molybdenum, tungsten or a combination of several other active chemical agents. The base metal catalysts are selective and operate in the specified temperature range. The major drawback of the base metal catalyst is its potential to oxidize SO2 to SO3; the degree of oxidation varies based on catalyst chemical formulation. The quantities of SO3 which are formed can react with the ammonia carryover to form various ammonium-sulfate salts.
Zeolite catalysts are aluminosilicate materials which function similarly to base metal catalysts. One potential advantage of zeolite catalysts is their higher operating temperature of about 970° F. (521° C.). These catalysts can also oxidize SO2 to SO3 and must be carefully matched to the flue gas conditions.
Precious metal catalysts are generally manufactured from platinum and rhodium. Precious metal catalysts also require careful consideration of flue gas constituents and operating temperatures. While effective in reducing NOx, these catalysts can also act as oxidizing catalysts, converting CO to CO2 under proper temperature conditions. However, SO2 oxidation to SO3 and high material costs often make precious metal catalysts less attractive.
As is known in the art, the concern about ammonia slip is not particularly a matter of costs of ammonia. The problem with ammonia slip is that it is increasingly unacceptable to the utility customer. Ammonia slip is a precursor to air heater fouling and direct PM2.5 emissions at the stack. It can even affect the salability of the fly ash for use in cement.
For coal fired boilers the principal problem arises from the reaction of ammonia with SO3 to form ammonium bisulfate. Ammonium bisulfate is a salt of a strong acid and weak base and is therefore acidic. Ammonium bisulfate has a relatively high dew point (approximately 350° F. to over 450° F.), as shown in FIG. 1. The melting point of ammonium bisulfate is about 297° F. So, any surface temperatures in the air heater hotter than about 297° F. and colder than the ammonium bisulfate dew point will attract deposits of acidic, liquid ammonium bisulfate. This acidic sticky substance will accumulate fly ash and produce deposits that are difficult to remove.
Currently SCRs are typically operated at low ammonia slips (e.g., less than or equal to about 2 ppm). However, with increasing ammonia slip various undesirable compounds will be generated potentially causing problems in downstream equipment and/or increased stack opacity.
Another problem associated with ammonia slip involves the particulate control device (e.g., an electrostatic precipitator). For example, problems have been observed with ammonia evolving from fly ash collected in the hoppers of the particulate collection device and subsequently used as fillers in cement. Eastern bituminous coal ashes tend to be acidic and therefore are unlikely to give off an ammonia odor, particularly in the face of the fact that the threshold odor concentration of ammonia is about 17 ppm. However, if these ashes accumulate ammonia under acid conditions, they could easily reverse the reaction when exposed to the alkaline conditions in cement.
The final fate of ammonia is perhaps the most problematic of all. If ammonia proceeds all the way to the wet scrubber in its vapor phase, then as soon as the flue gas is quenched to below about 180° F., ammonium bisulfite will form due to the presence of SO2 and water vapor. This ammonium bisulfite will form as a submicron aerosol that will not be captured in the wet scrubber. It will be discharged as a fine PM2.5 particulate and will persist for several miles downwind as a visible plume. For example, one ppm of ammonium bisulfite aerosol produces an obscuration of about 1% across a path length of 10 feet.
Given the above, a need exists for a method that provides for increased removal efficiency of NOx without increasing the amount of ammonia slip, and without suffering, for example, from the drawbacks of ammonium bisulfate formation, ammonia laden fly ash, and ammonium bisulfite formation.